Rework the way `tool_hugehelp.c` is included in builds.
After this patch, with `./configure` and CMake `tool_hugehelp.c` is only
compiled when building with manuals enabled. With manuals disabled this
source file is not used anymore. The method is similar to how
8a3740bc8e implemented `tool_ca_embed.c`.
`./configure` always generates it as before, otherwise the build fails.
- winbuild: rework to not need `buildconf.bat`, but automatically use
`tool_hugehelp.c` if present (e.g. when building from an official
source tarball) and enable `USE_MANUAL` accordingly.
- `buildconf.bat`: after dropping `tool_hugehelp.c` generation, the only
logic left was `cp Makefile.dist Makefile`. This allowed to launch
winbuild builds via GNU Make in a Git repo. Drop this option together
with the batch file.
- build `libcurltool` without `USE_MANUAL` macro to exclude the manual
and the dependence on the generator commands. Drop relying on
`UNITTESTS` for this purpose.
Follow-up to 96843f4ef7#16068
- `src/mkhelp.pl`: include `tool_hugehelp.h` before using `USE_MANUAL`
to have it set in `config-*.h` builds with source tarballs created
with manual but without zlib.
Closes#16081
